1. Field of the Invention
The present invention relates to a process for surface treatment of a stainless steel component for semiconductor manufacturing apparatus, and more particularly to a process for surface treatment to form on the surface of stainless steel a film which exhibits outstanding corrosion resistance to highly corrosive halogen gases such as HCl, Cl.sub.2, and HF.
2. Description of the Prior Art
The semiconductor manufacturing technology has recently reached a level where the high integration of elements requires that interconnects be spaced with a precision of the order of submicron. Such highly integrated elements are vulnerable to foreign substances no matter how they are small. Even a minute particle or bacterium will cause a short circuit which leads to defectives. For this reason the production of semiconductors needs extremely pure water and gases. To ensure the high purity of gases, it is necessary to minimize their contamination with impurity gases (such as moisture vapor) and fine particles originating from the surface of the piping and reaction chamber.
The gas piping for semiconductor manufacturing apparatus is conventionally made of austenitic stainless steel (such as Type 304L and Type 316L) because of its good weldability and corrosion resistance. Usually, it has its surface smoothed by electropolishing so as to reduce the adsorption area and thereby to reduce the adsorption and desorption of impurity gases. There have been proposed other technologies to this end. For example, Japanese Patent Laid-open No. 87760/1989 discloses a component designed to reduce the amount of gas released from the surface by means of an amorphous oxide film which is formed by heating in an oxidative gas atmosphere after electropolishing. Japanese Patent Laid-open No. 161145/1988 discloses a stainless steel pipe in which is extremely reduced the amount of non-metallic inclusions which give off fine particles and permit the adsorption and desorption of impurities. Japanese Patent Laid-open No. 59524/1993 discloses a stainless steel component with its surface layer coated by an oxide film (20-150 .ANG. thick) composed mainly of Cr.sub.2 O.sub.3 which needs less time for baking.
The above-mentioned stainless steel components are satisfactory when used as piping for non-corrosive gases such as oxygen and nitrogen. However, their surface is subject to corrosion by highly corrosive halogen gases such as HCl, Cl.sub.2, and HF, because it is as thin as 150 .ANG. or less no matter whether it is composed mainly of Fe.sub.2 O.sub.3 (which is poor in corrosion resistance) or Cr.sub.2 O.sub.3 (which is superior in corrosion resistance). Corrosion products adsorb and desorb gases to lower the purity of the feed gas. In addition, corrosion products (as metal chloride) in the form of fine particles are also the source of contamination.
Consequently, the semiconductor industry, in which there is a growing tendency toward the high degree of integration, has aroused an increasing demand for the material which has outstanding corrosion resistance to halogen gases. An example of such materials is high-Ni alloy (Hastelloy) which is superior in corrosion resistance to Type 304L and Type 316L. However, it is very expensive and yet it is not completely corrosion-resistant.